In the following, specific reference will be made to a logarithmic amplifier, although the invention is basically applicable to all amplifier circuits in which the characteristics fo a component, which determines the ratio fo two quantities, is used of form an input signal to an output signal that is dependent upon said characteristics.
A logarithmic amplifier is used in various fields of technology, amount others in connection with loudness regulators of audio apparatus. In such apparatus, a logarithmic amplifier serves to match the adjustment stroke to the subjective loudness sensation of the human ear.
In general, diodes or bipolar transistors are used for logarithmic amplifiers, since a diode (just like a bipolar transistor connected to operate as a diode) has a current-voltage characteristics corresponding to an exponential function, i.e., the diode current changes in about exponential manner with linearly increasing diode voltage. Thus, with an applied current, a diode voltage results corresponding approximately to a logarithm function, i.e., the inverse function an exponential function.
In a logarithmic amplifier designed as an integrated circuit (IC), an input current is usually fed into an input current circuit in which a diode (or a transistor) is provided.
Connected in parallel to this input current circuit is a reference current circuit in which a constant current source and also a diode (or a transistor, respectively) are provided, with the two components in the two current branches having as identical characteristic values as possible, in particular a defined ratio of the active component areas. Between the respective input nodes (e.g., anodes) of the diodes, a differential voltage results on the basis of the current density ratio. This differential voltage is amplified e.g., by means of a differential amplifier in order to obtain the logarithmic output signal.
A problem with such amplifiers is the dependency of the output signal on the temperature in the IC chip. Depending on the chip temperature, output signals of different magnitude are obtained for a given input current.
There are various temperature compensation measures known. With the aid of a component external to the chip, such as e.g., an ohmic resistor having a defined temperature coefficient, a temperature-dependent signal is obtained which, by suitable manners in terms of circuit technology, is processed such that the output signal of the integrated logarithmic amplifier is temperature-independent.
The wiring of a chip with external components is undesirable on principle since such additional components require considerable space on the circuit board. In addition thereto, the expenditure in manufacture is increased.
Instead of an external resistor, it is also possible to make use of a chip-internal resistor. However, in most of the integrated bipolar circuits there is no component available having a suitable temperature coefficient. To the contrary: in designing integrated circuits, efforts are made to keep possible temperature coefficients as low as possible. In order to be able to use only small temperature coefficients for temperature compensation of a logarithmic amplifier, there are complex circuits necessary.
With all known temperature compensations for logarithmic amplifier, the conformity between the temperature coefficient of the amplifier itself and the temperature coefficient of another component employed for temperature compensation is by no way ideal.
A temperature-compensated amplifier circuit for logarithmic amplification is known from JP-3-6906 A, herein incorporated by reference. As a first component, a diode is provided in the input current circuit, and in the reference current branch connected in parallel thereto, there is also provided a diode. The input signal fed to the input current circuit determines the current intensity delivered by a constant current source connected to the reference current branch. A differential amplifier forms of the voltages at the diodes a differential signal which is used for temperature compensation. Temperature compensation is carried out with the aid of a differential amplifier the non-inverting input of which is fed with the differential signal of the diode voltages and the inverting input of which is connected to a variable voltage source whose voltage value is dependent on the voltage on the diode in the input current circuit.
The publication DD 215 222, also incorporated herein by reference reveals a logarithmically operating current-voltage converter circuit in which, for conversion of the input current to the output current, a non-linear component is disposed in the feedback branch of an OP amplifier. For formation of a difference and temperature compensation, the output of this arrangement has connected thereto two also non-linear components in two parallel branches with an associated current source each. The two parallel branches are connected to the inverting and non-inverting input, respectively, of an output operational amplifier, with a linear attenuation member being disposed in one of said branches, With suitable dimensioning of the components, a temperature-dependent amplification of the temperature-dependent differential voltage formed on the non-linear components, i.e., a temperature compensation, takes place.